South Plainfield is a borough in northern Middlesex County, in the U.S. state of New Jersey. The borough is situated on the border with Union County in the Raritan Valley region, within the New York metropolitan area. As of the 2020 United States census, the borough's population was 24,338, its highest decennial count ever and an increase of 953 from the 23,385 recorded at the 2010 census, which in turn reflected an increase of 1,575 from the 21,810 counted in the 2000 census.